The present invention relates to a thermal recording material.
Typical thermal recording materials comprise a support such as a sheet of paper, synthesized paper, film or plastic. The support is coated with a coating solution. The coating solution comprises an electron-donative, colorless dye precursor which is normally colorless or light-colored, and an electron-accepting developer such as phenolic compounds. The dye precursor and the developer are separately ground into fine particles and mixed together, to which a binder, a filler, a sensitizer, a lubricant and other additives are added. In response to heating through a thermal head, a thermal pen, a laser beam or the like, the dye precursor instantaneously reacts with the developer to provide visible recording images. Such thermal recording materials have been applied in a wide range of fields including measuring recorders, printers for computer terminals, facsimile devices, automatic ticket vending machines, bar-code labels, etc. The quality requirement of the consumer for the thermal recording materials has been sophisticated as the recording devices have been diversified and had higher performances. For example, it has been required to (a) offer high-density and clear developed images with a smaller amount of thermal energy to increase recording speed and (b) have good storability involving light resistance, heat resistance, water resistance, oil resistance, and plasticizer resistance.
Thermal papers have more opportunities to be compared with plain papers than before with spreading application of a method of recording data on the plain paper such as an electrophotographic method and an ink-jet recording method. For instance, preservability of the resultant images on a thermal recording material is required to be comparable in quality to those recorded with toner. On the other hand, preservability of non-recorded portions (background portions of the paper) (hereinafter, referred to as background preservability) is required to be as close in quality as plain paper. The background preservability against heat (100.degree. C. or higher) or plasticizers is particularly required.
For the background preservability against heat, JP-A-4-353490 (the term "JP-A" as used herein means Japanese Patent Laid-open, or an "unexamined" published Japanese patent application) discloses a thermal recording material in which background or whiteness and density of recorded portion are not deteriorated under a high temperature environment of around 90.degree. C. More specifically, the background of this thermal recording material has a density of some 0.11, measured by a Macbeth densitometer, after processed in a drier at 95.degree. C. for 5 hours. This result is relatively fair but is not in a satisfactory level. In addition, the conventional recording materials with the phenolic developer are insufficient in the heat resistance, so that it is impossible to laminate a film or the like through heat sealing or thermal laminating on the surface of the material subjected to the thermal recording.
The background preservability can be improved in thermal materials comprising a thiourea compound rather than the phenolic one. The thiourea compound is essentially different in structure from a phenolic compound commonly used as a developer. For example, JP-A-58-211496, JP-A-59-184694, JP-A-60-145884, JP-A-61-211085, JP-A-5-4449, and JP-A-5-185739 disclose thermal recording materials comprising a thiourea compound as the developer which the materials are superior in the background preservability (such as heat resistance, water resistance, and plasticizer resistance) and preservability of recorded images. Of these, thiourea compounds disclosed in JP-A-58-211496, JP-A-59-184694, and JP-A-61-211085 are monourea compounds having only one phenylthiourea structure Ar-NH-C.dbd.S-NH. These compounds have no superiority to the phenolic developer in view of the heat resistance, which is a major cause of their unsuccessful use in practical applications.
On the other hand, JP-A-60-145884 discloses diphenyl-bis-thiourea, diphenyl-p-phenylene-dithiourea, and diphenyl-m-phenylene-dithiourea. The first one has two phenylthiourea structures which are directly linked to each other. The remaining two each has two phenylthiourea structures linked through a phenylene group. These thiourea compounds are, however, used along with a color developing enhancer and are thus disadvantageous in the heat resistance. In addition, these thiourea compounds have some disadvantages in the heat resistance of the recorded and background portions even if they are used alone.
JP-A-5-185739 discloses a thermal recording material in which a bisthiourea compound is used as the developer to provide improved resistance to ethanol and plasticizers. The bisthiourea compound is used, however, along with a sensitizer. Accordingly, it is impossible to achieve the heat resistance at 100.degree. C. or higher.
JP-A-5-4449 discloses that recorded images can be stabilized by means of adding, as a third compound, a bisthiourea compound to the color developing composition comprising a dye precursor and a salicylic acid developer. However, this compound can provide neither the heat resistance at 100.degree. C. or higher nor a "reversible recording" characteristic, which allow repeated cycle of recording and erasing, when being prepared according to a method disclosed in the specification.
The reversible recording is an important factor in the field of current thermal recording. More specifically, tremendous efforts have been made to improve the thermal recording materials and such improvements result in rapid increase in consumption amount of the recording material involving in establishment of networks as well as spread of facsimile and copying machines. This means that increased volume of thermal recording papers has been used, which is responsible for current social problems of refuse disposal. A thermal reversible recording material has thus been of interest that permits recording and erasing repeatedly as an approach to this problem.
For example, JP-A-3-230993, and JP-A-4-366682 disclose thermal reversible recording materials whose state changes reversibly from transparent to opaque due to the given temperature. The recording materials for opaque appearance are, however, inferior in clarity and brightness. In addition, color recording is not available in some of the thermal reversible recording materials.